(i) Field of the Invention
This invention relates to a process for preparing aromatic alkenes, especially butadienes, and novel bis-dienes.
(ii) Description of Prior Art
The most common method used in the past for the synthesis of diarylbutadienes involves the Wittig reaction using phosphonium salts (cf R. N. McDonald & T. W. Campbell, J. Org. Chem., 24, 1969 (1959)).
Two methods for the synthesis are outlined below. The reactions proceed in high yield but require the use of molar amounts of the expensive triphenylphosphine in the reaction which is converted to triphenylphosphine oxide. It also requires the synthesis of benzyl or phenylpropenyl halides as intermediates. ##STR1##
In a series of papers and patents Siegrist (A. E. Siegrist, Helv. Chim. Acta, 50, 906 (1960), et seq; A. E. Siegrist, P. Liechti, H. R. Meyer and K. Weber U.S. Pat. No. 4,158,099, June 12, 1979) demonstrated a facile synthesis of stilbenes by the condensation of anils of aromatic aldehydes, which are readily accessible from the condensation of the aldehydes with aniline, with methyl-substituted aromatic compounds.
Siegrist prepared several thousand stilbenes from heterocyclic compounds as well as hydrocarbons and in many cases yields of over 90 percent were obtained. The reaction proceeds under relatively mild conditions in aprotic dipolar solvents such as N,N'-dimethylformamide with bases such as potassium hydroxide or potassium t-butoxide because the coproduct in the reaction is aniline and hence the high basicity of the reaction mixture can be maintained.
An attempt has been made to synthesize polymeric stilbenes using the anil reaction (G. Kofsmehl and A. Yarridjanian, Makromol. Chem. 182, 3419 (1981)). The polymers were not thoroughly characterized, however it is unlikely that high molecular weight materials were formed since the reaction is not a high yield reaction and the insoluble polymers when formed would precipitate from the reaction mixture before high molecular weight could be obtained. The synthesis of bifunctional monomers or oligomers containing stilbene moieties by this method is possible. For example, one could synthesize either a dihydroxystilbene or a stilbene dicarboxylic acid by this route and these materials could be used in the synthesis of thermotropic aromatic polyesters which could subsequently be crosslinked with molecules such as maleic anhydride as in the recent work described by Calundann et al (cf. G. W. Calundann, H. A. A. Rasoul and H. K. Hall, Jr., U.S. Pat. No. 4,654,412 (Mar. 31, 1987)).